1. Field of the Invention
The present invention relates to an image pickup apparatus and a control method therefor in which a mechanical shutter and an electronic shutter are used in combination to control an amount of exposure, and more particularly, to an image pickup apparatus and a control method therefor in which strobe light emission is controlled while an image is being picked up.
2. Description of the Related Art
As an image pickup apparatus, a digital camera is known. In some digital camera of single-lens reflex type, photographing is performed while using a focal plane shutter (hereinafter, referred to as mechanical shutter) and an electronic shutter in combination (see, for example, Japanese Laid-open Patent Publication No. 11-41523).
In such a digital camera, a rear curtain of a shutter mechanism of the camera is constituted by the mechanical shutter. At the time of photographing, the electronic shutter is driven to perform scanning to accumulate charges in pixels of an image pickup device of the camera before the rear curtain is operated or traveled. Hereinafter, the electronic shutter and the mechanical shutter will be sometimes referred to as the electronic front curtain shutter and the mechanical rear curtain shutter, respectively.
The image pickup device is implemented by a CMOS sensor, for example. In that case, charge accumulation is started after pixels are reset on a per pixel basis or on a per scanning line basis, i.e., after scanning that zeros the accumulated charge amount of each pixel is performed. Then, signal reading scanning is made on a per pixel basis or on a per scanning line basis after lapse of a predetermined time period from the start of the charge accumulation, whereby an electronic shutter is realized. Hereinafter, such charge accumulation start scanning will be referred as the reset scanning.
In a digital camera having the mechanical rear curtain shutter used in combination with the electronic front curtain shutter, the image pickup device of the camera is light-shielded by the mechanical rear curtain shutter during or after the reset scanning. Then, reading scanning is performed to read charges accumulated in the pixels of the image pickup device. The reset scanning must be made in a scanning pattern matching a travel characteristic of the mechanical rear curtain shutter.
At the time of strobe photographing, the mechanical front curtain shutter is used in the digital camera of single-lens reflex type, and completion of travel of the mechanical front curtain shutter is detected by a photo reflector or by a mechanical switch. Then, a strobe emission signal is generated based on a result of the detection.
The above also applies to a case where the digital camera is set to a photographing mode in which the electronic front curtain shutter is usable. This is because the curtain shutter is not provided with a mechanical shutter mechanism, so that a strobe emission signal cannot be generated at completion of the shutter travel. Thus, even in the photographing mode in which the electronic front curtain shutter is usable, strobe photographing is not performed by using the electronic front curtain shutter, but performed by using the mechanical front curtain shutter.
As described above, with the electronic front curtain shutter whose completion of travel cannot be detected by a photo reflector or by a mechanical switch, a problem is posed that the strobe emission signal cannot be generated, so that the strobe cannot be emitted with accuracy.